nc6: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/bin/nc6 - from package: netcat6

You may install the required package with command apt-get install {package}
Error in other languages:
nc6: bevel nie gevind nie
nc6: comandă negăsită
nc6: comando não encontrado
nc6: commande introuvable
nc6: command not found
nc6: käsku ei ole
nc6: không tìm thấy lệnh
nc6: komanda nerasta
nc6: Kommando nicht gefunden.
nc6: kommandot finns inte
nc6: komut yok
nc6: nie znaleziono polecenia
nc6: níor aimsíodh an t-ordú
nc6: no se encontró la orden
nc6: no s'ha trobat l'ordre
nc6: opdracht niet gevonden
nc6: parancs nem található
nc6: perintah tidak ditemukan
nc6: príkaz nenájdený
nc6: příkaz nenalezen
nc6: команда не найдена
nc6: командата не е открита
nc6: 命令找不到
nc6: コマンドが見つかりません